Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 3 Nov 2022 23:00:53 +0900
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>, linux-mm@kvack.org,
        David Gow <davidgow@google.com>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Guenter Roeck <linux@roeck-us.net>,
        Andy Shevchenko <andriy.shevchenko@intel.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH 2/6] slab: Remove special-casing of const 0 size
 allocations
Message-ID: <Y2PJlfSDij7rDoW4@hyeyoo>
References: <20221101222520.never.109-kees@kernel.org>
 <20221101223321.1326815-2-keescook@chromium.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221101223321.1326815-2-keescook@chromium.org>
Precedence: bulk

On Tue, Nov 01, 2022 at 03:33:10PM -0700, Kees Cook wrote:
> Passing a constant-0 size allocation into kmalloc() or kmalloc_node()
> does not need to be a fast-path operation, so the static return value
> can be removed entirely. This is in preparation for making sure that
> all paths through the inlines result in a full extern function call,
> where __alloc_size() hints will actually be seen[1] by GCC. (A constant
> return value of 0 means the "0" allocation size won't be propagated by
> the inline.)
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96503
> 
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Cc: linux-mm@kvack.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/slab.h | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index e08fe7978b5c..970e9504949e 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -562,17 +562,13 @@ void *kmalloc_large_node(size_t size, gfp_t flags, int node) __assume_page_align
>  #ifndef CONFIG_SLOB
>  static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
>  {
> -	if (__builtin_constant_p(size)) {
> +	if (__builtin_constant_p(size) && size) {
>  		unsigned int index;
>  
>  		if (size > KMALLOC_MAX_CACHE_SIZE)
>  			return kmalloc_large(size, flags);
>  
>  		index = kmalloc_index(size);
> -
> -		if (!index)
> -			return ZERO_SIZE_PTR;
> -
>  		return kmalloc_trace(
>  				kmalloc_caches[kmalloc_type(flags)][index],
>  				flags, size);
> @@ -592,17 +588,13 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
>  #ifndef CONFIG_SLOB
>  static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t flags, int node)
>  {
> -	if (__builtin_constant_p(size)) {
> +	if (__builtin_constant_p(size) && size) {
>  		unsigned int index;
>  
>  		if (size > KMALLOC_MAX_CACHE_SIZE)
>  			return kmalloc_large_node(size, flags, node);
>  
>  		index = kmalloc_index(size);
> -
> -		if (!index)
> -			return ZERO_SIZE_PTR;
> -
>  		return kmalloc_node_trace(
>  				kmalloc_caches[kmalloc_type(flags)][index],
>  				flags, node, size);
> -- 
> 2.34.1

Looks good to me.

Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

-- 
Thanks,
Hyeonggon